1. Field of the Invention
This invention relates to a video camera and, more particularly, to a HDTV-type video camera.
2. Description of the Prior Art
When the number of pixels of a CCD image pick-up device increases, the resolution in reproduction is improved. However, as the number of pixels increases, the problem with defects of pixels grows. An existing system corrects defects of pixels in a CDS circuit on the part of a camera, or in a defect correcting circuit as disclosed in the U.S. Pat. No. 4910598.
In FIG. 1, a reference sync signal REFSYNC is sent from a terminal 10 to a camera control unit (hereinafter called CCU) 11. The reference sync signal REFSYNC is sent from the CCU 11 to a signal generator 14 in the camera 12 through a transmission cable 13 which connects the camera 12 to the CCU 11.
The signal generator 14 generates a timing signal for each circuit element on the basis of the reference sync signal REFSYNC sent from the CCU 11. The timing signal is sent to a CCD driver 15. A transfer clock is made in the CCD driver 15, and it is sent to the CCD image pick-up device 16. In response to the transfer clock, the CCD image pick-up device 16 is driven, and outputs an image pick-up signal. The output signal from the CCD image pick-up device 16 is sent to a CDS circuit 17, which samples it and holds the sampled value. An output signal from the CDS circuit 17 is sent to a process circuit 18 which includes a .gamma. correcting circuit, a knee circuit and so forth. An output signal from the process circuit 18 is supplied to the CCU 11 via the cable 13.
Defects may occur in pixels of the CCD image pick-up device 16. In defective pixels, the preceding sampled value is output from the CDS circuit 17. Then the defects of the CCD image pick-up device 16 is corrected.
On the other hand, a high definition TV system using 1125 scanning lines (hereinafter called HDTV) is nearly brought to practical use. The number of pixels of a CCD image pick-up device of a HDTV video camera is much more than that of an NTSC television system. Therefore, the number of defective pixels also increases. As a result, if correction of the defects of the CCD image pick-up device 16 relies on the CDS circuit 17 on the part of the camera as shown in FIG. 1, a heavy load is imposed on the CDS circuit 17. A solution to avoid this problem is to provide a defect correcting circuit on the part of the CCU 11. More complicated signal processing is possible on the part of the CCU 11 than on the part of the camera.
As discussed above, in order to correct defects on the part of the CCU 11, it is necessary that a signal for obtaining address information of the CCD image pick-up device 16, that is, a signal for obtaining information having the same time base as the transfer clock given to the CCD image pick-up device 16 in the camera 12, be sent from the camera 12 to the CCU 11. It may be possible to use the sync signal contained in the video signal as the information.
However, since the frequency of the transfer clock is much higher than that of the sync signal, if defects of pixels is corrected by the CCU 11, the accuracy of the time base is insufficient, only with the sync signal added to the video signal. In addition, stabilities of the CCD driver 15 and the process circuit 18, GEN-lock accuracy in case of a change in length of the cable 13, and stability of the signal generating circuit, etc. in the CCU 11 must be taken into consideration. Therefore, the accuracy of the time base is insufficient, only with the reference sync signal REFSYNC.